1. Field of the Invention
The present invention provides a cerium oxide powder, a method for producing the powder, and the use thereof.
2. Description of the Background
Owing to its extraordinary properties as an abrasive and catalyst material, there are numerous patent applications and journal articles on the production of cerium oxide powders. Cerium oxide is generally produced by the calcination of cerium hydroxides or cerium carbonates. The calcined oxides are then ground and sieved. Another way of synthesising cerium oxide consists in hydrothermal synthesis, as described e.g. in U.S. Pat. No. 5,389,352. Here, cerium(III) salts are converted oxidatively, under the influence of temperature and pressure, to cerium oxide, which crystallises out in the form of fine particles. Of particular interest are spray pyrolysis methods, in which a cerium oxide precursor, generally in the form of an aerosol, is oxidised in a flame. A method of this type is described e.g. in EP-A-1142830.
Pratsinis et al. in J. Mater. Res., vol. 17, pages 1356-1362 (2002), describe the production of cerium oxide powder of high crystallinity by flame spray pyrolysis of cerium acetate solutions in solvent mixtures of acetic acid, iso-octane and 2-butanol by oxidation using oxygen in an oxygen/methane flame. The solvent mixture is essential for the production of a cerium oxide powder that is free from, or substantially free from, coarser particles. Acetic acid alone leads to powders with a coarse portion. Particularly advantageous in achieving a fine particle size powder that is free from coarse portions is an increase in the cerium oxide precursor flow rate and a reduction in the oxygen flow. The average particle size is derived from the XRD spectrum. Especially the feed of iso-octane, with its high combustion enthalpy and a high evaporation/combustion rate, is said to reduce the proportion of coarser particles. It is stated that the specific surface of the cerium oxide particles, which can be between 101 and 240 m2/g, can be altered by varying the ratio of cerium oxide precursor and oxygen. An increase in the precursor feed rate with constant oxygen flow is said to lead to an increase in the particle size, an increase in the oxygen flow with constant precursor feed rate to a reduction in the particle size.
Only an average primary particle size can be taken from the document. No information is given as to how these are distributed. From the TEM photograph on page 1357 it can be seen that the primary particles of the fine portion have fused together into aggregates, which have a high degree of aggregation. It is known to the person skilled in the art that precisely aggregates of this type can be dispersed only with difficulty. Especially when the cerium oxide is used as an abrasive in dispersions, this can lead to scratches on the surface to be polished and to uneven removal. In addition, dispersions containing cerium oxide particles of this type often exhibit only low stability with respect to sedimentation.
The pore sizes and their distribution are an important feature when cerium oxide powders are used in catalytic processes. The cerium oxide powder produced by Pratsinis et. al. has a bimodal pore size distribution, wherein 10% of the pores are smaller than 10 nm.
Furthermore, the process described only enables small quantities of cerium oxide (range approx. 10 g/h) to be produced and is thus not very economical. No options are given for scaling up the process. Furthermore, the use of methane and iso-octane as combustible gas leads to a high proportion of carbon in the reaction mixture. Thus, the risk of cerium oxide being contaminated with carbon is also increased.
U.S. Pat. No. 5,772,780 claims a dispersion containing cerium oxide for the polishing of semiconductor substrates, wherein the cerium oxide is present in the form of aggregated crystalline particles with a crystallite size of no more than 30 nm. No production process for the cerium oxide particles and the cerium oxide-containing dispersion is disclosed.
U.S. Pat. No. 6,420,269 claims an abrasive containing a cerium oxide grain, which is obtained by oxidation of a water-insoluble, trivalent cerium salt dispersed in water. The cerium oxide has a specific surface of between 50 and 500 m2/g and a density of 0.8-1.3 g/ml. The size of the cerium oxide primary grain is no more than 10 nm, the size of the cerium oxide secondary grain, which is formed by aggregation from the primary grains, is less than 1 μm. The secondary grains have a perimeter that is free from edges and has an angle of less than 120°. The cerium oxide grain has a main signal in the X-ray powder diffractogram with a half width of no less than 0.4° and no more than 0.5°. In addition, it is an essential feature that the material is not completely crystallised. This is said to display advantages in the polishing of semiconductor substrates.
U.S. Pat. No. 6,221,118 claims a dispersion containing cerium oxide particles of low crystallinity, which consists of primary particles with an average diameter of 30 to 250 nm. The cerium oxide particles in the dispersion have an average diameter of 150 to 600 nm. The cerium oxide particles are obtained by combustion of cerium carbonate.
In the German patent application number DE 10251029.6 of 02.11.2002, a pyrogenically produced cerium oxide powder with a specific surface of 15 to 200 m2/g is claimed, which consists of a coarse portion of crystalline, non-aggregated particles with an average diameter of between 30 and 200 nm and of a fine portion in the form of aggregates of fine crystalline, fused primary particles with an average aggregate diameter of 5 to 50 nm. It is produced in that at least one compound capable of being converted into cerium oxide by oxidation is fed into a flame, which is produced from a hydrogen-containing combustible gas, preferably hydrogen, and an excess of air or an air/oxygen mixture, and is reacted there. The discharge velocity of the fluid droplets from the atomizer unit into the reaction chamber is greater than 70 m/s here and the velocity of the reaction mixture in the reaction chamber is between 0.5 and 2 m/s. The flame has an oxygen excess, expressed as a lambda value, of at least 1.6.
From WO 01/36332 the production of cerium oxide particles is known. The cerium oxide particles are produced in a high-temperature reaction zone, generally a flame, by oxidation of cerium salt solutions, which are fed into the reaction zone in the form of fine droplets. In this way, aggregates are obtained which consist substantially of approximately spherical primary particles. At least some of these aggregates have a cenospherical structure with an aggregate size of 1 to 20 μm. In addition, the cerium oxide composition may also contain aciniform (grape-like) aggregates with an average aggregate diameter of less than 500 nm. A cerium oxide composition in which the cenospherical aggregates predominate is preferred. A disadvantage of this cerium oxide composition is that the cenospherical aggregates are unstable and friable, and can therefore disintegrate in an irregular manner in applications in which energy has to be applied, e.g. shear energy, during incorporation into dispersions. Furthermore, the fragments obtained after dispersion can lead to an undesirable, non-uniform, unpredictable particle size distribution in the dispersion.
On the one hand, there remains a keen interest in cerium oxide, and on the other hand it becomes clear that even small changes in the way in which the reaction is conducted lead to a material with different properties.
In particular, dispersions containing cerium oxide play an important part in the polishing of semiconductor substrates (CMP, chemical-mechanical polishing). With the increasing miniaturization of components, the polishing operation is of central importance.